October 26, 2007

ZeroLink, Rest in Pieces

One thing that made me happy, and was greeted with applause at WWDC, was the news that ZeroLink is gone in Leopard. The linker is now fast enough to make ZeroLink unnecessary.

So what was so bad about ZeroLink? The first problem wasn’t actually ZeroLink’s fault – it was Apple making ZeroLink on by default for new Xcode projects. A common refrain heard from new Mac programmers was “I made a Cool Little Cocoa App, sent it to a friend it won’t work for him. What’s wrong?” ZeroLink.

ZeroLink performed its magic by demand-loading object files, and those object files weren’t included in the application bundle until several Xcode revisions later. So sending an App to a friend (or a reviewer, or a cow-orker) would mean sending them a broken app. Then the programmer would ask for help on a mailing list or IRC, get told to turn of ZeroLink, and then rebuild and re-send. In fact, the Feenode #macdev FAQ has a special section just for ZeroLink.

This was embarrassing and demoralizing, especially for someone new to the platform. New, small projects didn’t benefit from ZeroLink anyway, and you have to have a pretty large program before the time savings is noticeable. Someone with a project that big will have someone intimate with Xcode who would know about ZeroLink, so need to have it on by default.

The second problem was that ZeroLink introduced instability, with its frequent refrain being “I’m getting weird crashes”, which are fixed after turning off zero link. I got paid for a full day’s consulting tracking down a strange C++ operator new overload. The daily smoke test built and ran great, but development versions crashed and burned in weird places. We finally tracked the main difference between the developer builds and the automated builds: ZeroLink. Turned it off, rebuilt, and life was happy ever after. This particular app (which was freaking huge) was broken down into lots of frameworks, so link time wasn’t that much longer than with ZeroLink. (Because of this framework breakdown, I was able to pull some command-line tricks to prevent unnecessary recompilation of other parts of the system, and greatly reduced my turnaround time on a TiBook500 vs the G5-weilding full-timers, but that’s a story for another day.)